A speed control system which allows the driver to operate the accelerator pedal less frequently has been heretofore proposed, for example, in Published Unexamined Japanese Utility Model Application No. 57-5932. In such systems, when the vehicle goes into a different terrain, e.g., when it starts to climb a hill after traveling a flat road, the control system makes it unnecessary for the driver to operate the accelerator for controlling the vehicle speed. For this purpose, the control system sets the vehicle speed according to the position of the accelerator pedal operated by the driver and controls the opening of the throttle valve for the internal combustion engine to attain the set speed.
This known speed control system controls the opening of the throttle valve according to the difference between the actual speed and the set speed. That is, the system relies on a PID control. However, for PID control, a delay is introduced in the response time of the control system and the control accuracy is low.
In an attempt to solve the foregoing problems, the present inventor has tried to apply a linear control theory to the aforementioned speed control system for improving the responsiveness of the control system and for stabilizing the control accuracy. This method of design consists in finding a physical model describing the behavior of the controlled system, in this case the whole vehicle including the internal combustion engine, and determining a control law based on the model. However, if the speed control system is designed by this method of design, then the whole control system must be redesigned whenever a part of the driving system of the vehicle, such as the engine or the transmission, is modified. Thus, the design work is cumbersome to perform. As an example, for a second vehicle that has the same engine as the engine of a first vehicle but is equipped with a different power train, a new control law applying to the power train as well as to the engine must be determined. In other words, the same control law is not applicable to different vehicles even when they are equipped with the same engine.
Another method of design is known for the case where the linear control theory is applied to a controlled system, such as a vehicle, for which a linear relationship does not hold between the control input, or the throttle opening, and the control output, or the vehicle speed, as encountered where the vehicle speed is controlled. In this method, the operating state of the vehicle is divided into several regions in each of which a linear relation holds between the control input and the control output. A physical model is found and a control law is determined for every region. Whenever the vehicle enters a different region, the control law is switched to another. Therefore, when the speed control system is designed by this method of design, a control law must be set for every region of the vehicle operating state. However, when the speed is actually controlled, the control operation becomes complex because the control law must be switched.